Tree invasion in a semi-arid savanna in Zimbabwe : seedling recruitment of Acacia karroo

In this thesis Chirara reports on his study on the competitive interaction between savanna grasses and young tree seedlings of Acacia karroo, from hereon indicated as ' Acacia seedlings' . Acacia is one of the tree species that dominates savanna grassland in situations of overgrazing (bush encroachment). The discussion is whether this invasion is predominantly caused by the decreasing light competition or water competition of grasses in overgrazed situations. So, Chirara studied: -Acacia seedling survival in the field under different grazing intensitie -Acacia seedling survival in an experimental set up with different grass clipping regimes -Water availability in relation to grass clipping regimes and the effect on it for Acacia seedling survival -Interaction between water and light limitation for Acacia seedling survival -Effect of defoliation of Acacia seedlings on survival. Surprisingly, low grass cover as well as a high grass cover facilitated Acacia seedling establishment. Most probably shade of grasses reduces evaporation and hence the water demand of the very young tree seedling. This will effect establishment rather than survival on the long run. Young Acacia seedlings develop very quickly a long thin taproot in search for water deeper in the soil. This enables the seedling to survive during the dry season. In deep shade (less than 6% of full sunlight) seedling growth is very much hampered and independent of soil water level. These situations of less than 6% are not exceptional in the field. However, in the slightly higher light levels, water shortage is much more determining seedling growth. Depressed seedlings due to water shortage develop less quickly their long taproot and that can lower the probability for seedling survival in the dry season or dry spells during the wet season. Grazing up to 90% of the Acacia seedling finally did not effect survival and growth. The seedling recovers very quickly. In conclusion, grass cover effects seedling establishment and growth. However, considering the plasticity of the allocation pattern and especially the ability to produce a deep taproot early in its life cycle, enables the Acacia seedling to accommodate to stress caused by grass competition. Competition is important in suppressing Acacia seedlings, but not the only mechanism

Phenotypic and genotypic analyses to guide selection of reverse transcriptase inhibitors in second-line HIV therapy following extended virological failure in Uganda

Objectives We investigated phenotypic and genotypic resistance after 2 years of first-line therapy with two HIV treatment regimens in the absence of virological monitoring. Methods NORA [Nevirapine OR Abacavir study, a sub-study of the Development of AntiRetroviral Therapy in Africa (DART) trial] randomized 600 symptomatic HIV-infected Ugandan adults (CD4 cell count <200 cells/mm3) to receive zidovudine/lamivudine plus abacavir (cABC arm) or nevirapine (cNVP arm). All virological tests were performed retrospectively, including resistance tests on week 96 plasma samples with HIV RNA levels ≥1000 copies/mL. Phenotypic resistance was expressed as fold-change in IC50 (FC) relative to wild-type virus. Results HIV-1 RNA viral load ≥1000 copies/mL at week 96 was seen in 58/204 (28.4%) cABC participants and 21/159 (13.2%) cNVP participants. Resistance results were available in 35 cABC and 17 cNVP participants; 31 (89%) cABC and 16 (94%) cNVP isolates had a week 96 FC below the biological cut-off for tenofovir (2.2). In the cNVP arm, 16/17 participants had resistance mutations synonymous with high-level resistance to nevirapine and efavirenz; FC values for etravirine were above the biological cut-off in 9 (53%) isolates. In multivariate regression models, K65R, Y115F and the presence of thymidine analogue-associated mutations were associated with increased susceptibility to etravirine in the cABC arm. Conclusions Our data support the use of tenofovir following failure of a first-line zidovudine-containing regimen and shed further light on non-nucleoside reverse transcriptase inhibitor hypersusceptibility

Grass defoliation affecting survival and growth of seedlings of Acacia karroo, an encroaching species in southwestern Zimbabwe

Two experiments were conducted, one in the field and the other in the greenhouse, to investigate the effects of the intensity and frequency of grass defoliation on the survival and growth of Acacia karroo seedlings. In the greenhouse, seedlings growing with heavily clipped grasses had higher biomass production than those competing with moderately clipped grasses. Root/shoot ratios were higher in treatments with unclipped grasses. There was a negative relationship between grass root production and A. karroo biomass production. The field experiment was carried out in two paddocks, one previously heavily-grazed and the other lightly-grazed. Grazing in both paddocks was simulated by artificial defoliation. Generally more A. karroo seedlings emerged under lightly defoliated treatments. Clipping frequency had a strong effect (P = 0.066) on the survival of emerged seedlings during the wet season. There were no differences in survival rate at the end of the dry season. Though grass defoliation was shown to enhance seedling growth under controlled conditions, no evidence was found to suggest that seedling establishment during the first year is influenced by the intensity of grass defoliation.Keywords: Acacia karroo; botany; bush encroachment; grass; grass defoliation; grazing; growth; seedling establishment; seedlings; survival; Zimbabwe; biomass production; clipping; defoliation; setaria incrassata; treesAfrican Journal of Range & Forage Science, Vol. 15(1 & 2), pp. 41–4

Impact of the N348I Mutation in HIV-1 Reverse Transcriptase on Nonnucleoside Reverse Transcriptase Inhibitor Resistance in Non-Subtype B HIV-1▿

We investigated the effect of N348I alone and with M184V on nonnucleoside reverse transcriptase inhibitor (NNRTI) drug susceptibility and replicative capacity in B and non-B HIV-1 isolates. N348I reduced the susceptibility to all NNRTI drugs across subtypes. The replication capacity of all viruses in a variety of cell lines was impaired by N348I. Interestingly, the N348I and M184V double mutation compensated for the reduced NNRTI drug susceptibility observed in the N348I single mutant and marginally improved viral replicative capacity